This invention relates to the coating of major surfaces of elongated strip articles, such as sheet metal strip and the like. More particularly, it relates to apparatus and methods for continuously and automatically controlling the thickness of a layer of paint or other liquid coating material applied to a strip major surface.
By way of specific illustration, detailed reference will be made herein to the coating of sheet metal in greatly elongated strip form, as used for making siding panels cladding exterior building walls, it being understood, however, that the invention in its broader aspects embraces other coating of other types of articles and surfaces, as well as detection and maintenance of film thickness.
In the production of siding panels for metal strip, at least one major surface of the strip is first given a protective and decorative coating of paint, and the strip is thereafter formed and cut into individual panels. As described, for example in U.S. Pat. No. 4,675,230, the disclosure of which is fully incorporated herein by this reference, there is shown an apparatus and procedure for applying paint or the like coating material to an elongated strip article using a coating head having an open-sided slot to which coating material is supplied under pressure, and a support, such as a roll around which the strip is advanced past the head for receiving from the slot a layer of paint metered between the head and the strip, wherein a load is continuously exerted on the head during operation for urging the head against the applied paint layer on the strip, so as to maintain a uniform metering gap between the head and the coating surface. The load may be exerted by devices such as air cylinders acting on the head and capable of adjustment to vary the magnitude of the load for different coating operations.
In coating operations of the general type described in U.S. Pat. No. 4,675,230, desired coating thickness uniformity requires maintenance of a metering orifice of invariant aperture; i.e., the spacing between the coating head and the coated strip major surface downstream of the slot must remain constant despite variations in the thickness of the strip being coated. It is desirable to be able to translate the coating head relative to the major surface of the article being coated, so as to be able to maintain the desired constant orifice width, i.e., distance between the head surface and the major surface of the article being coated, so as to compensate for nonuniformities in the article major surface topography. For example, if the article which is being coated is a sheet of aluminum or other metal, the sheet may have local thickness variations longitudinally or laterally across the sheet, which will vary the topography of the sheet surface. Therefore, in order to maintain a uniform coating thickness across the sheet (i.e., constant orifice width), in spite of sheet surface topography variations, the coating head must be conformed to the sheet strip's surface topography.